The Directory Replication Service Remote Protocol (DRSR) is the RPC mechanism domain controllers use to exchange Active Directory data and keep directory replicas synchronized. It carries user, group, computer, and other directory objects between controllers, and under the right permissions it can also expose sensitive authentication material.
DRSR matters because it is a legitimate trust channel that attackers can abuse. In techniques such as DCSync, an adversary with replication rights can impersonate a replication partner and request directory secrets without first compromising a domain controller directly. Defenders reduce this risk by tightly limiting replication permissions such as DS-Replication-Get-Changes-All, auditing who can use them, and monitoring directory-service activity for unusual replication requests.